This invention relates to a stratified charge type combustion process for an internal combustion engine, and an internal combustion engine utilizing same, in which an inflammable mixture and residual exhaust gases are stratified and burnt in a combustion chamber or in a cylinder of an internal combustion engine, the objective being to reduce the quantity of nitrogen oxides (NO.sub.x) in the exhaust gases without accompanying increase in the quantity of hydrocarbon therein or an increase in fuel consumption.
In general, when the air-fuel ratio A/F of the mixture supplied to the internal combustion engine is about 16, the quantity of nitrogen oxides, NO.sub.x emitted is a minimum. An air-fuel ratio larger than the above value (i.e., a leaner mixture charge) may reduce the quantity of NO.sub.x emitted. In a diesel engine, an increase in the excess air ratio aids in reduction in the quantity of NO.sub.x emitted or generated. An excess air ratio of .lambda.=1 corresponds to the theoretical air-fuel ratio of about 15.
An excess air ratio .lambda.(&gt;1) corresponds to a lean mixture charge condition for an engine of a spark ignition type. However, air-fuel ratios of over 18 to 19 result in an increase in the quantities of hydrocarbon and carbon monoxide emitted. This is due to the fact that if the air-fuel ratio A/F is increased, the flame propagating speed is reduced and the combustion temperature also drops, thus reducing the quantity of NO.sub.x emitted. However, the flame is extinguished in the close vicinity of the wall surface of the combustion chamber, so that combustion in this region is incomplete, leaving hydrocarbons, incompletely-burned hydrocarbons and carbon monoxide at the wall.
For reduction in the quantity of NO.sub.x, there has been proposed an exhaust gas recirculation system (EGR), in which part of the exhaust gases is introduced through an intake pipe into a combustion chamber to be mixed with intake air or the intake mixture charge.
According to the exhaust gas recirculation system (EGR system) the exhaust gases are distributed substantially uniformly throughout a combustion chamber or a cylinder. This lowers the maximum combustion temperature in the central portion of the combustion chamber, thus reducing the quantity of NO.sub.x effectively. However, in the peripheral portion (near the wall surface) of the combustion chamber, there is little reduction in the quantity of NO.sub.x, produced but there is a remarkable increase in the quantities of HC and CO produced.
In the case of combustion at a relatively high air-fuel ratio (high excess air ratio in the case of a diesel engine), even in the absence of exhaust gas recirculation, the flame reaches the wall surface of a combustion chamber at the final stage of combustion, and the wall surface of the combustion chamber is not heated so that the combustion temperature drops. Consequently, when exhaust gas recirculation is applied, the combustion temperature drops further so that the flame is extinguished before reaching the wall surface of the combustion chamber. Accordingly, the greater the quantity of recirculating exhaust gases introduced, the more difficult will be combustion at a high air-fuel ratio (a super-lean mixture combustion). This is also followed by an increase in the quantity of fuel unburned and an increase in fuel consumption.
Analysis of the results of experiments on the relationship between the quantities of NO.sub.x, HC and CO produced in a cylinder and the gas temperature therein reveals that the temperature in the central portion of the combustion chamber (combustion gas temperature) has an important bearing on the quantity of NO.sub.x produced, while scarcely affecting the production of HC and CO. With regard to a diesel engine and a spark ignition type engine provided with a sub-chamber respectively, a tendency similar thereto is noted in the combustion gases which are firstly injected from a sub-chamber as well as those in the central portion of a combustion chamber. In addition the gas temperature in the vicinity of the wall surface of a combustion chamber, where a flame reaches in the later half or in the final stage of combustion has an important relation to the quantities of HC and CO, while exerting little influence on that of NO.sub.x.
FIGS. 1A, and 1B represent the aforesaid relationship. The maximum gas temperature (.degree.K.) is represented by the abscissa in both FIGS. 1A and 1B and the weights or amounts of NO.sub.x, HC and CO exhausted are represented by the ordinate therein, FIG. 1A shows the weight of the aforesaid gases exhausted from the central portion of a combustion chamber, while FIG. 1(b) shows the weight of the aforesaid gases exhausted from the vicinity of the wall surface of a combustion chamber.